The invention relates to a method of manufacturing a colour display tube having a magnetic quadrupole post-focusing mask, which mask is formed from a sheet of a magnetisable material having rows of apertures, and which mask is magnetised so that cyclically a north pole, a south pole, a north pole and a south pole are formed along the circumference of each aperture.
The invention also relates to a colour display tube made by the method.
A method of manufacturing a colour display tube having a magnetic quadrupole post-focusing mask is disclosed in British Patent Publication 2074782A, corresponding to U.S. Pat. No. 4,428,736, issued Jan. 31, 1984 and U.S. Pat. No. 4,513,272, issued Apr. 23, 1985. The object of magnetic post-focusing is to increase the transmission of the mask. In tubes without post-focusing, a very large part, for example 80 to 85%, of the electrons is intercepted by the so-called shadow mask. By using magnetic post-focusing, however, the apertures in the mask can be enlarged, since as a result of the focusing action in the apertures the electron spots impinging on the screen are considerably smaller than the apertures, so that sufficient space is present between the electron spots of the various electron beams to avoid their overlapping onto adjacent phosphors.
In the known tube, the mask is formed by a magnetisable sheet which has a large number of aperture and which is magnetised so that cyclically a north pole, a south pole, a north pole and a south pole are present along the circumference of each aperture. The sheet may be manufactured from a ferro-magnetic material or from a non-ferro-magnetic material on which a layer (or layers) of magnetisable material has (or have) been provided. A magnetic quadrupole lens is present in each of the apertures, which lens focuses the electron beam in one direction and defocuses it in a direction at right angles thereto to produce an elongated spot.
British Patent Specification 2074782A discloses two methods of magnetising a mask sheet. In one method, the magnetisation is carried out providing two mutually substantially perpendicularly oriented set of parallel conductors, one set on either side of the apertured mask sheet.
The conductors of each set are connected in series in a serpentine or meanderline fashion. The conductors are secured in predetermined positions on a block of synthetic resin. The spacing between the conductors is such that when placed against a mask sheet, pairs of conductors extend between adjacent rows of apertures. Thus when a potential difference is applied, current passes in opposite directions through the conductors of each pair. By suitably arranging the sets of conductors, a magnetic quadrupole is formed around each aperture in the mask in which oppositely directed poles are situated at an angle of 90.degree. from each other and at an angle of approximately 45.degree. with the longitudinal direction of the spot of the electron beam. Although this method could be used to magnetise quadrupole lenses in the whole mask sheet in one operation, in reality this suggestion is impractical, as will be illustrated by the following example.
The crosssectional size of the conductors is determined by a number of factors, two of which are the pitch of the rows of apertures and the spacing between adjacent rows. An exemplary cross section size of the conductor is 100 .mu.m.times.300 .mu.m and the spacing between the conductors of each pair is 50 .mu.m. An exemplary magnetising current is of the order of 600 amps and to avoid fusing the conductors, the current is applied in the form of pulses of say 20 .mu.s duration. An R.C.L. discharge circuit is used to provide these pulses and such a circuit requires physically large capacitors. A large number of conductors requires an equally large number of capacitors, causing the circuit to be unwieldy physically. A more practical approach would be to magnetise the small areas of the mask sheet in a step and repeat method. However, a step and repeat method produces magnetisation problems at the interfaces of each step. Since the magnetising current required is dependent on the thickness of the material, decreasing the pitch does not lead to a proportionate decrease in thickness of the material and so not to a decrease in magnetising current. Additionally, the development of heat in the magnetising devise becomes worse. In small display tubes, the material of the mask is not necessarily any thinner because the mask has to be of a certain rigidity.
In the other method disclosed in Specification 2074782A, the magnetisation is carried out by providing two mutually substantially perpendicularly oriented sets of permanently magnetised strips and arranging them with one set on either side of the mask sheet. The strips have alternate oppositely directed poles on the side facing the mask plate. The pitch of the strips is such that a strip extends between the apertures of each row and other strip extends over a row of apertures. A coil is provided around at least a part of the mask sheet with the strips provided thereagainst. A decaying magnetic alternating field is generated in the coil at the area of the mask sheet, which magnetic alternating field initially drives the magnetisable material of the sheet on both sides of the hysteresis curve into saturation. This hysteresis curve is superimposed on the constant magnetic field generated by the magnetic strips, which field actually induces the quadrupoles. This method also has a number of disadvantages in that the magnetic field induced by a step and repeat method is substantially constant over the entire area of the mask sheet. Also this other method is not suitable for magnetising small and/or high definition mask sheets because decreasing the pitch of the apertures in the mask sheet requires the permanently magnetised strips to be disposed closer and closer to each other while remaining physically separate. In practice such bunching of the strips is difficult to achieve.
Both of these known magnetising methods produce quadrupole lenses of substantially the same strength over the entire area of the shadow mask, and one disadvantage of this is that a focusing ring is produced. A focusing ring arises because with post-focusing, the beamlets transmitted by the apertures in the mask sheet are in focus over a ring-shape area of the screen, while the portions of the image inside and outside this ring-shaped area are out of focus to varying degrees. This is because the beamlets are focused more strongly in the peripheral regions (as a result of the beamlets passing obliquely through their respective lens field) than in the central region, where the beamlets pass substantially perpendicularly to their respective lens field. The presence of the focusing ring is perceptible to the eye, and as a compromise measure to reduce its effect, the beam spots at the screen are defocused slightly. This may lead to perceptible landing (colour purity) problems. However, such a compromise measure might be unnecessary if the focusing ring problem could be solved.
Another disadvantage of having a constant magnetic field produced by orthogonally related strips is that spot rotation occurs at the corners of the mask due to the electron beams passing obliquely through the apertures. Although this problem could be avoided by adapting the hole shape and rotating the quadrupoles, these known magnetising methods are too inflexible to allow such an adaption.
An object of the present invention is to overcome the above-mentioned disadvantages in the manufacture of colour display tubes having magnetic quadrupole post-focusing masks.